problem with keil flash file system

hi to all,
i am creating a something like file explorer with lpc1788,with keil v5.11a as compiler.
keil flash file system work correctly and good when i want read any file from memory card,but when i want write to it,some thing happened that surprised(and confused) me!!!!
please look at on function of my code(that will copy a file from usb flash disk to memory card) :

unsigned char Copy_File_Flash_To_Sd(char *Dest,char *Source)
{
        FILE* myDest;
        FRESULT fResult;
        BYTE res = 0x00;
        char copyRes = 1;
        UINT tmp = 1;
        FIL mySource;

        //checking that source file exist or no
        if(f_stat(Source + 4,0x00) != FR_OK)
                return 0;

        myDest = fopen(Dest,"w");

        //checking for errors when opening file
        if(myDest == NULL)
        {
                char i = 0;
                //re initlie flash fs(try in three time)
                for(i = 0;i < 3;i++)
                {
                        if(finit(0) == 0)
                                break;
                }

                //re open file
                myDest = fopen(Dest,"w");
                if(myDest == NULL)
                        return 0;
        }

        //opening source file for read(+4 skim "U0:\" part of path)
        if(f_open(&mySource,Source + 4,FA_READ) != FR_OK)
        {
                //closing opened file in sd card
                fclose(myDest);

                //reporting that proccess was faild
                return 0;
        }

        //creating a delay
        msDelay(50);

        //hitherto all thing is true

        //reading all byte of source file
        while(1)
        {
                //reading one byte from flash disk
                fResult = f_read(&mySource,&res,1,&tmp);

                //check that file_read,read any thing or no
                if(tmp != 0 || fResult)
                {
                        /*            ..::**** problem is here ****::..
                        problem  is here,that when i want to write res to opened file in
                        sd card only write 4KB of file in it and then fputc return error!!!
                        Surprisingly when change res in below line with '15'(an const int
                        number) it write it correctly(!!!).
                        note that res has valid value,i check it.
                        */
                        if(fputc(res,myDest) == EOF)
                        {
                                copyRes = 0;
                                break;
                        }
                }
                else
                        break;
        }

        //closing opened files
        for(res = 0;res < 3;res++)
        {
                if(f_close(&mySource) == FR_OK)
                        break;
                else if(res == 2)
                        copyRes = 0;
        }

        for(res = 0;res < 3;res++)
        {
                if(fclose(myDest) == 0)
                        break;
                if(res == 3)
                {
                        return 0;
                }
        }

        if(copyRes == 1)
                return 1;
        else
                return 0;
}

now any one can help me to fix this problem,any one has any experience with keil flash file system?????
Excuse me for long question,i did not know how make it shorter.

regards,alireza sd.